Caterpillars.

Whilst I was pruning a silver birch tree in the garden for V this morning, she noticed these caterpillars on one of the branches so I immediately fetched my camera and took this photo.

However, I am ashamed to say that I cannot identify the creatures. I'm pretty sure sure they're not butterfly larvae even though they bear a superficial resemblance to Small Tortoiseshell caterpillars. Anyway they are not usually seen on plants other than nettles. These must be moth larvae and there are too many moths for me to be able to guess. Perhaps someone can tell me.

The ant on the left of the two upper caterpillars looks pretty ominous and he did fetch one or two of his pals later in the day. Perhaps they are just interested in the caterpillars' secretions.